Silvio
Well-known member
expressel;15636]I found out that the zener don't provide the neccessary current for the discrete mosfet driver.
Also 470n is not enough for the bootstrap capacitor. I used 10uF
The image is too small to see
You can make an auxiliary winding to support the current needed for the drive circuit, As start up you can use a tip50 transistor or even a mosfet which can regulate the output through the base or gate of the device. This in turn charges a bulk capacitor of say 1000uF. This will hold enough energy for a few cycles until the aux winding takes over to supply this voltage. To make something like this work you need to say start up with a regulated voltage of 12v then putting a diode after the regulator to isolate. Another voltage from the auxiliary will supply say 15v from a 7815 regulator this will will also be tied to the positive of the drive circuit. After start up the voltage will only be supplied from the aux winding and the start up circuit will automatically switch off due to it seeing a higher voltage at the output.
Bootstrap capacitors are rather tricky at times. Too large is no good as it does not have enough time to charge up between each cycle, too small on the other hand will not hold enough energy to supply the needed current to switch the gates. This will also depend on the frequency of oscillation. At over 100Khz you will need to see the voltage and rise time on the gates to see that there is enough energy with the minimum possible capacitance. You must use a good capacitor here possibly polypropylene to avoid premature failure. You can put 2 or 3 in parallel to get the correct value. These take quite a beating and must be of good quality.
Also 470n is not enough for the bootstrap capacitor. I used 10uF
The image is too small to see
You can make an auxiliary winding to support the current needed for the drive circuit, As start up you can use a tip50 transistor or even a mosfet which can regulate the output through the base or gate of the device. This in turn charges a bulk capacitor of say 1000uF. This will hold enough energy for a few cycles until the aux winding takes over to supply this voltage. To make something like this work you need to say start up with a regulated voltage of 12v then putting a diode after the regulator to isolate. Another voltage from the auxiliary will supply say 15v from a 7815 regulator this will will also be tied to the positive of the drive circuit. After start up the voltage will only be supplied from the aux winding and the start up circuit will automatically switch off due to it seeing a higher voltage at the output.
Bootstrap capacitors are rather tricky at times. Too large is no good as it does not have enough time to charge up between each cycle, too small on the other hand will not hold enough energy to supply the needed current to switch the gates. This will also depend on the frequency of oscillation. At over 100Khz you will need to see the voltage and rise time on the gates to see that there is enough energy with the minimum possible capacitance. You must use a good capacitor here possibly polypropylene to avoid premature failure. You can put 2 or 3 in parallel to get the correct value. These take quite a beating and must be of good quality.
